Trap mouse on mouseover if Lix window is active

This reverts commit c33f2b6.

Problem with that c33f2b6: If you left by accident, then quickly
returned, you had to click inside Lix, thus sometimes cancelled the
replay unwanted if you chose the left mouse button.

Second problem: When you moved the mouse fast, possibly even within Lix,
the mouse got untrapped and you saw the OS cursor in addition to the Lix
cursor. It's possible that this fast mouse movement left Lix under the
hood, but it didn't feel like it.

src/hardware/mouse.d

@@ -165,6 +165,8 @@ void consumeAllegroMouseEvents()
             break;
 
         case ALLEGRO_EVENT_MOUSE_ENTER_DISPLAY:
+            if (hardware.display.displayActive)
+                _trapMouse = true;
             // Nepster 2016-06: When entering the display, the ingame
             // cursor should jump to where we entered.
             // This would sometimes trigger when the hardware mouse is close